This invention relates to the manufacture and use of a thulium doped calcium fluoride, CaF.sub.2 (Tm), thermoluminescent material, having relatively widely separated glow maxima and, to a method for measuring the amount of radiation to which an object or person carrying a dosimeter made from the material, has been exposed.
A thermoluminescent material is one which emits or is emitting light as it is heated. A thermoluminescable material is one which may be rendered thermoluminescent -- but the terms are used more or less interchangeably in this specification, and the term thermoluminescent is generally favored because it is conventionally used in the art.
Workers in a radiation environment, as a precaution against being exposed to more radiation than can be safely tolerated, either intentionally or otherwise, carry a device such as a badge, which records the amount of radiation to which the badge, and the person wearing it, have been exposed.
Thermoluminescable powders in the form of thermoluminescable dosimeters are preferred materials for such a badge because they absorb radiation by trapping it. At a much later time, say at the end of a week, the accumulated absorbed radiation is measured by heating a thermoluminescable dosimeter in a badge to a predetermined temperature at which it emits light of an intensity correlatable to the amount of radiation accumulated. Instruments, known as readers or readout systems, are available in which an exposed dosimeter is heated and the amount of light emitted is measured in scale units. The scale units are correlated to indicate the intensity of radiation in roentgens to which the material was exposed. After a thermoluminescent dosimeter is "read" it may be reused. Though existing instruments may be used to evaluate an exposed CaF.sub.2 (Tm) dosimeter, the method of evaluating the novel dosimeters is unique.
Though the conventional method of reading an exposed thermoluminescent dosimeter gives acceptable results, the properties of known thermoluminescent materials are such that they are difficult to read in the presence of neutrons. Another drawback is that they fade in the environment and this "fade" is difficult to evaluate because of a lack of data on the temperature history of the dosimeter being read. Even where the presence of neutrons is not a problem, and the dosimeter has a known temperature history, conventional materials do not permit an accurate determination of an accumulation of plural, very low readings. The instant CaF.sub.2 (Tm) provides a solution to all the foregoing problems associated with conventional materials.
Numerous thermoluminescent materials are known, many of which are disclosed in U.S. Pat. No. 3,532,777. Among the known materials are lithium fluoride doped with magnesium and titanium and calcium fluoride doped with lithium cryolite and lithium fluotitanate. Neither material has well-separated glow maxima.
Calcium fluoride is also disclosed in U.S. Pat. No. 3,203,899 as a laser or maser host material for numerous dopands of rare earth metals and transition elements, singly or in combination. Such a doped host material is taught to provide a material for amplifying microwave energy by stimulated emission of radiation (maser) and for amplifying light energy by stimulated emission of radiation (laser). A useful maser or laser material is characterized by an essentially instantaneous transfer of energy which amplified the signal introduced into the material; in other words, the lifetime of the trapped energy is exceedingly short, in the order of microseconds or nanoseconds. In contrast, the lifetime of a "trap" in a thermoluminescent material is generally several hours of days, and preferably at least 1 week. Thus it will be apparent to one skilled in the art that a material suitable for a maser or laser will be wholly unsuited as a thermoluminescent material for a dosimeter, and of course, one would not expect to look for, or find, a thermoluminescent material among laser or maser materials.
It should also be noted that much effort has been devoted to the formulation of thermoluminsecent materials and a large number of host materials have been doped or activated with a comparably large list of dopands or activators. Exceedingly few of the materials have been found to possess desirable thermoluminescent properties. The work has served to emphasize that there is no predictable interchangeability of activators for host materials. Neither does there seem to be any guide as to the manner in which activators work. Thus, though there is not much question as to the ability to incorporate at least a small amount of any desired activator in almost any preselected host material, there is no reasonable expectation that a useful thermoluminescent material will result.
The novel thermoluminescent material of this invention, and the method of utilizing thulium doped calcium fluoride, provides a long-awaited solution to difficult problems of dosimetry.